1. Field of the Invention
The invention relates to a transformer and a core assembly thereof, and in particular relates to a transformer and a core set thereof for enhancing leakage inductance.
2. Description of the Related Art
Due to a demand for thinner monitors, liquid crystal displays (LCDs) have merits for reducing the thickness and having a high-quality frame. Consequently, the LCDs have rapidly replaced CRT monitors. A backlight module of an LCD monitor comprises a cold cathode fluorescent lamp (CCFL), driven by high voltage, to serve as a light source for the backlight system of the LCD. In general, the CCFL is driven by an inverter, which comprises a drive circuit and a high-voltage transformer.
Although LCD monitors are much thinner than CRT monitors, size requirements for LCD monitors continue to grow. Thus, the length of the CCFL must be increased. The leakage capacitance is therefore increased due to the increased LCD monitor size. To improve entire efficiency and the balance of tube current, matching the leakage inductance and the leakage capacitance is performed to decrease the damage of the transformer. Accordingly, the leakage inductance is necessarily increased for matching up with the increased leakage capacitance.
Referring to FIG. 1A and FIG. 1C, a conventional transformer 1 comprises a bobbin 10 and two cores 20. The bobbin 10 comprises a primary winding area 11 and two secondary winding areas 12. The primary winding area 11 is at the center of the bobbin 10, and the two secondary winding areas 12 are at two sides of the primary winding area 11, and furthermore, the bobbin 10 has a hollow portion 13 therein. The two cores 20 are E-shaped. Each core 20 has a protrusion 21 at the middle and are inserted respectively into the hollow portion 13 from two opposite sides of the bobbin 10. A cover 14 covers the bobbin 10. The cover 14 is made of the insulated materials, and the shape of the cover 14 fits the shape of the bobbin 10. The cover 14 protects a primary coil (not shown) and two secondary coils (not shown), both of which are wound around the primary winding area 11 and the two secondary winding areas 12. The cover 14 comprises an opening 141 in the axial direction to allow the protrusions 21 of the two cores 20 to pass therethrough. Additionally, a gap A (as shown in FIG. 1B) of the conventional transformer 1 is formed between two arms 22 of the cores 20.
The number of the coils or the distance between the primary coils and the secondary coils can adjust the leakage inductance of the conventional transformer 1. However, the space of the winding area 11 and 12 and the length of two cores 20 be increased. Also, it increases the volume of the transformer 1. If the coil diameter is decreased to substitute for changing the available space for winding areas 11 and 12, the temperature will increase. Thus, the conventional transformers need to be improved.
Referring to FIG. 2A and FIG. 2C, a conventional transformer 2 comprises a bobbin 10, a first core 30 and a second core 40. The bobbin 10 comprises a primary winding area 11 and two secondary winding areas 12. The primary winding area 11 is at the center of the bobbin 10, and the two secondary winding areas 12 are at two sides of the primary winding area 11. Additionally, the bobbin 10 has a hollow portion 13 therein, and is covered by a cover 14. The cover 14 is made of the insulated materials. The shape of the cover 14 fits the shape of the bobbin 10. The cover 14 protects a primary coil (not shown) and the two secondary coils (not shown), both of which are wound around the primary winding area 11 and two secondary winding areas 12. The cover 14 comprises an opening 141. The first core 30 is I-shaped, and disposed in the hollow portion 13 of the bobbin 10 and the opening 141 of the cover 14. The second core 40 is frame-shaped, and mounted on the cover 14. The second core 40 comprises protrusions with U-shaped cross sections at both ends. Each protrusion comprises two protrusions 41. Additionally, a gap B (as shown in FIG. 2B) of the conventional transformer 2 is formed between the first core 30 and the protrusions 41 of the second core 40.
The size of the gap B, the number of the coils or the distance between the primary coils and the secondary coils can adjust the leakage inductance of the conventional transformer 2. It is therefore understood that conventional transformer 2 has the same drawbacks as the conventional transformer 1. Accordingly, the conventional transformer 2 needs to be improved.